Remove meshes from Gazebo

[ahcorde]

🎉 Enhancement

Summary

This PR allows to remove a meshes when an entity is removed or the remove UserCommand is called.

Related with https://github.com/ignitionrobotics/ign-gui/issues/208

Checklist

• [x] Signed all commits for DCO
• [ ] Added tests
• [ ] Added example and/or tutorial
• [ ] Updated documentation (as needed)
• [ ] Updated migration guide (as needed)
• [x] codecheck passed (See contributing)
• [x] All tests passed (See test coverage)
• [ ] While waiting for a review on your PR, please help review another open pull request to support the maintainers

Note to maintainers: Remember to use Squash-Merge

[chapulina]

@ahcorde , what's the status of this PR?

[chapulina]

This requires https://github.com/ignitionrobotics/ign-common/pull/222/.

We're not actively working on these PRs, so I'm removing them from Fortress' beta.